diff --git a/mypy/fastparse.py b/mypy/fastparse.py
index 8b6eb3d28b67..f68d1b0279d0 100644
--- a/mypy/fastparse.py
+++ b/mypy/fastparse.py
@@ -736,7 +736,7 @@ def is_star2arg(k: ast35.keyword) -> bool:
     # Num(object n) -- a number as a PyObject.
     @with_line
     def visit_Num(self, n: ast35.Num) -> Union[IntExpr, FloatExpr, ComplexExpr]:
-        if getattr(n, 'underscores', None) and self.pyversion < (3, 6):
+        if getattr(n, 'contains_underscores', None) and self.pyversion < (3, 6):
             raise FastParserError('Underscores in numeric literals are only '
                                   'supported in Python 3.6', n.lineno, n.col_offset)
         if isinstance(n.n, int):
diff --git a/mypy/test/testcheck.py b/mypy/test/testcheck.py
index b795bd5dcfaf..8cbca4262d17 100644
--- a/mypy/test/testcheck.py
+++ b/mypy/test/testcheck.py
@@ -74,7 +74,7 @@
 if 'annotation' in typed_ast.ast35.Assign._fields:
     files.append('check-newsyntax.test')
 
-if 'underscores' in typed_ast.ast35.Num._fields:
+if 'contains_underscores' in typed_ast.ast35.Num._fields:
     files.append('check-underscores.test')
 
 
diff --git a/test-data/unit/check-underscores.test b/test-data/unit/check-underscores.test
index 51926c3adf1e..a1d88cbf268f 100644
--- a/test-data/unit/check-underscores.test
+++ b/test-data/unit/check-underscores.test
@@ -12,4 +12,5 @@ x = 1000_000_  # E: invalid token
 # flags: --fast-parser --python-version 3.6
 x: int
 x = 1000_000
+x = 0x_FF_FF_FF_FF
 y: str = 1000_000.000_001  # E: Incompatible types in assignment (expression has type "float", variable has type "str")